Magnetron sputter deposition is a well-known method of applying coatings onto substrates, especially large glass sheets and continuous films or webs of polymeric substrate materials, intended for various applications, such as solar control window films, optically enhanced holograms, transparent anti-static packaging, large scale solar energy collectors and solar cells, electromagnetic interference shielding, etc.
In the accompanying drawings, FIG. 1 illustrates a conventional commercial coating line utilizing conventional planar magnetron sputtering cathodes. FIG. 2 illustrates schematically, in a slightly different setting and in a very simplified manner, the sputtering process. In brief, the coating material to be sputtered is mounted as a negatively biased target on the surface of the cathode element of the magnetron and a substrate material to be coated is moved along a path opposite the target. Conventional sputtering may occur upwardly, downwardly or horizontally, as desired, i.e., FIG. 2 may be viewed right side up, on its side, or upside down, all with equal effect.
In nonreactive sputtering, the sputtering chamber is evacuated, argon or other inert gas is introduced, the substrate is started in motion, the magnetron is energized, and the inert gas molecules begin a cascade ionization process and form between the target and the substrate a plasma cloud of ions and electrons which, in the core of the plasma cloud, have equal amounts of positive and negative charges. The positively charged ions at the plasma cloud edge, driven by the electric field, leave the plasma cloud and move toward the target surface and bombard the target material with high energy impact, thereby causing the target to disintegrate atom-by-atom and to be driven and redeposited atom-by-atom onto the substrate. The process produces a uniform and strongly adherent coating which is resistant to abrasion, peeling and cracking.
In reactive sputtering, one or more active gases capable of reacting with the target material are introduced into the sputtering chamber along with the inert gas. The active gas molecules react with the sputtered atoms of target material as they travel through the plasma cloud and impinge on the substrate thereby to form a desired compound coating on the substrate, e.g., an oxide or a nitride of the sputtered metal.
Inherently, the same chemical reaction takes place on the exposed surface of the target, which creates serious problems in terms of process control. Because the sputtering rate is usually substantially lower for the compound than the corresponding base metal, the total sputtering rate will be gradually reduced to a much lower level in the presence of the active species, commonly called "target poisoning". Also in most cases, the compound has very poor electrical conductivity and severe arcing problems inevitably develop. As the compound forms on the target surface, one or more localized points of low conductivity are formed and a static charge builds up eventually resulting in a high energy arc which, like a bolt of lightning, sinks a large current to a small area, evaporates the target at that point and causes vapor deposition onto the substrate of nonuniform droplets of much greater size than the sputtered atoms. This causes an unacceptably rough or bumpy coating on the substrate and disrupts the sputtering process. When severe arcing occurs, the power must be shut off to quench the arcing. This results in discontinuous production runs, loss of time and waste of materials. Arcing is especially difficult to control in reactive sputtering with oxygen, and in many cases makes reactive sputtering unmanageable and nonproductive.
In order to cope with the arcing problem, the industry relied for a time on RF sputtering to deposit nonconductive films. However, RF sputtering is too slow for profitable coating in large scale production.
Presently, the vacuum coating industry prefers to use rotary target systems such as those available from Airco under the trademark "C-MAG". In this system, as illustrated in FIG. 3, one or more rotatable target tubes are cooled internally with coolant so the cathode can handle high sputtering powers. As the target or targets rotate slowly, the segment of the cylindrical surface exposed to the plasma is sputtered and previously sputtered surface areas are rotated out of the reaction zone. When entering into the sputtering zone, previously formed nonconductive surface areas are cleaned by the sputtering process before they build up excessively and cause severe arcing. The self cleaning of the rotating target tube or tubes thus reduces severe arcing frequency and renders the process much easier to manage.
However, the rotary target tube system is not an efficient system for deposition of low melting point metals because the power and thus the sputtering rate must be maintained at a relatively low level to prevent melting of the target tube and dropping of metal out of the target. Deposition of low melting temperature metal compounds, e.g., oxides and nitrides of bismuth, tin, gallium and their alloys is especially troublesome because of the low melting point and poor thermal conductance.
Relative to another aspect of the invention, it is observed that the art includes disclosures of deposition processes involving the use of a wholly or partially liquid or molten target, namely, U.S. Pat. No. 3,799,862 to Krutenat, Japanese patent document 53-43905 and German patent publication 2528108 to Kausche et al. The Krutenat system cannot be used for reactive sputtering because the filament which is an integral part of the system would burn out in a reactive plasma. The Japanese publication employs a RF power supply to override a DC power supply to start and sustain the sputtering process, which for commercial production would be prohibitively expensive and inefficient. Kausche proposes a continuous supply of a large volume of liquid material to be sputtered, which likewise would be prohibitively expensive and inefficient. Moreover, none of these disclosures addresses either the arcing problem or the problem of reduced sputtering capacity due to the formation of a nonconductive blanket or crust of metal compound on the surface of the liquid metal once sputtering commences. Process control would therefore rapidly degenerate to an unmanageable condition.
To the best of the applicant's information and belief, there were no liquid or molten target sputtering processes in commercial use at the time of the present invention.